1949 — Ionization-Type Vacuum-Gauge Tube

An ionization-type vacuum-gauge tube designed for measuring gas pressures in high-vacuum systems and enclosures, featuring a hard-glass envelope and dual tungsten filaments.

Overview

The 1949 is a specialized ionization-type vacuum-gauge tube designed to determine gas pressures in vacuum systems and vacuum enclosures. It operates by ionizing residual gas molecules within the system and measuring the resulting positive ion current. The tube is capable of measuring extremely low pressures, with a useful sensitivity range extending below 0.1 micron of mercury (Hg), or less than $10^{-4}$ mm of Hg.

To ensure a long operating life, the 1949 is constructed with two independent tungsten filaments—one active and one spare. If the primary filament fails or becomes depleted, the spare filament can be connected to continue operation without replacing the entire tube.

Construction and Physical Dimensions

The 1949 is built with a T-16 hard-glass bulb and features Corning Code 772 Nonex hard-glass tubulation. This tubulation is designed to be sealed directly to the vacuum system under test.

The physical dimensions and mechanical features of the tube include:

• Maximum Tube Length: 11-1/2 inches to 11-3/4 inches (excluding flexible leads)
• Maximum Tube Radius: 2-3/16 inches
• Maximum Bulb Length: 5-1/8 inches
• Maximum Bulb Diameter: 2-1/16 inches
• Tubulation Diameter: 3/8 inch to 1/2 inch
• Terminal Leads: Four flexible leads (0.050-inch diameter, approximately 4-1/2 inches long) extending from the base, and one top lead (0.050-inch diameter, approximately 1-5/16 inches long) serving as the plate terminal.

Operating Position

The tube can be operated in either a vertical or horizontal position. When mounted vertically, the tubulation may point either up or down. When mounted horizontally, the tube must be oriented so that the internal stem press remains in a vertical plane.

Terminal Connections and Filament Ratings

The 1949 utilizes a five-terminal connection scheme. The four flexible base leads and the single top lead are connected as follows:

• Lead 1: Common lead to filaments
• Lead 2: Filament (Main or Spare)
• Lead 3: Filament (Spare or Main)
• Lead 4: Grid
• Top Lead: Plate (associated with the tubulation)

Filament Characteristics

The tungsten filaments are designed for low-voltage, high-current operation. The electrical characteristics for either filament operated alone are:

During degassing, the filament voltage should be kept as low as possible to prevent premature aging and maximize the overall life of the filaments.

Electrical Ratings and Operating Limits

The absolute maximum ratings for the 1949 define the safe limits for both normal operation and the degassing process. Operating the tube outside of these limits may cause permanent damage or alter the calibration of the gauge.

Typical Operation and Sensitivity

During typical operation, the plate is biased negatively to collect positive ions, while the grid is biased positively to accelerate electrons from the filament. These accelerated electrons collide with and ionize the residual gas molecules.

The sensitivity of the tube is defined as the ratio of plate current (ion current) to gas pressure at a specified grid current. For dry air, with a constant grid current ($I_c$) of 10 mA and a DC plate voltage ($E_b$) of -22.5 V, the typical operating parameters are:

(Note: 1 micron of Hg is equal to 0.001 mm of Hg).

Calibration and Curve Characteristics

The relationship between gas pressure and plate current is highly linear when plotted on a log-log scale across the pressure range of $10^{-7}$ to $10^{-4}$ mm of Hg. Operating at higher grid voltages (+110 V or +160 V) increases the ionization efficiency, shifting the calibration curves to higher plate currents for any given pressure and resulting in higher sensitivity.

Degassing Procedure

Adsorbed gases on the internal metal parts of the tube can release slowly over time, causing false pressure readings. To prevent this, the tube must be periodically degassed by heating the grid and plate electrodes. This is achieved by connecting the grid and plate together and applying a high voltage to draw electron current from the filament.

Typical degassing conditions are specified for both AC and DC methods:

During degassing, the combined dissipation of the grid and plate must not exceed the absolute maximum rating of 150 W, and the voltage applied to the tied elements must not exceed 650 V (DC or peak AC).

Technical Summary

The 1949 is an ionization-type vacuum-gauge tube featuring a T-16 hard-glass bulb with Corning Code 772 Nonex tubulation and a dual tungsten filament system (one active, one spare) rated at 5.0 V / 3.5 A. It is designed to measure gas pressures below 0.1 micron of Hg ($10^{-4}$ mm of Hg).

By operating with a negative plate voltage (-22.5 V) and a positive grid voltage (+80 V to +160 V), the tube establishes a linear relationship between gas pressure and ion plate current, providing sensitivities of 80 to 140 μA/micron at a constant grid current of 10 mA. The tube supports both AC and DC degassing methods up to a maximum combined electrode dissipation of 150 W to maintain measurement accuracy.